The present invention is directed to a RFID transponder and, more particularly, to a RFID transponder having an active load modulation unit and a method for supplying energy to the RFID transponder.
The ISO/IEC 14443 contactless smartcard infrastructure is optimized for relatively large antenna sizes (ISO-card ID1). However, more and more devices, like wearable devices, require smaller antenna geometries, which results in a much lower coupling coefficient to the reader antenna as compared when smartcard sized antennas are used. This lower coupling coefficient limits the power transmitted to small transponders, and more importantly, also limits the strength of return link signals from the transponders to a reader.
Conventionally, the return link signals of a passive transponder are generated by loading the magnetic field of the reader (load modulation, also called passive load modulation). This passive load modulation may be realized with a resistor/resistors or a capacitor/capacitors switched by the data signal loading the magnetic field. It also may be realized with a transistor acting as a resistor. However, if the signal received by the reader is below the reader's sensitivity, the transponder cannot be detected, read or written to by the reader.
To overcome this weak signal issue, “active load modulation” is used, such that instead of changing the chip input impedance by switching a resistor or a capacitor, the transponder actively transmits a signal back to the reader. This active transmission has to be frequency and phase synchronous to the reader carrier signal, only in this case the return signal looks like a load modulated signal to the reader, which is very important so that existing infrastructure can be used.
Since transponders using active load modulation consume more power than those using passive load modulation, the transponders need sufficient energy to perform the active load modulation. However, passive transponders using small antennas can't supply sufficient energy for active load modulation. Thus, a battery is used to store the energy needed to perform for active load modulation. However, embedding a battery in a transponder increases the size of the transponder, which defeats the ability to reduce the size of the transponder, especially for wearable, small form factor transponders. Further, for semi-passive transponders that have an assisted battery but do not use the battery to supply energy for active load modulation, using the battery to supply energy for active load modulation would require a bigger battery, thus increasing the size and cost of the transponders.
Accordingly, it is desired to have an energy supply solution for small form factor passive or semi-passive transponders.